In a digital communication system such as a private mobile communication system, M16QAM (multi-subcarrier 16 quadrature amplitude modulation, M=4), one of DMCA modulation method is used. The M16QAM transmits the 16QAM signal by using the multicarrier method, in which four subcarriers perform the transmissions at the same time. This communication method enables the transfer speed of 4K symbols/second for 64K bps (bits/second) data. "Digital Method MCA System Standard Regulations (RCR-STD-32)" is a document issued by the Electric Wave System Development Center Foundation, which discusses this subject. In this document the details of the modulation method, encoding method, sync/pilot insertion, fall/rise slot structural diagram, measurement method, and modulation-precision definitions are given.
The data is encoded in 16-bit units and then transmitted as shown in FIG. 14. This 16-bit data is split into four 4-bit segments and transmitted by the four subcarriers, ch. 1-4. Each subcarrier compresses the bandwidth using the roll-off=0.2 root Nyquist filter (bandwidth-compression filter). The interval between any two subcarriers is 4.5 KHz, and 18 KHz of bandwidth is used for the four channels. The four-channel subcarrier transmission system diagram is shown in FIG. 15 for reference.
The 4-bit unit data is split into two bits of I-component and two bits of Q-component as shown in FIG. 12. By orthogonally encoding these bits, the 4-bit data is expressed in 16-value encoded combinations. This symbol will hereinafter be referred to as the "reference information symbol (RIS)." Besides the data symbols, on the transmission side a sync symbol and pilot symbol are inserted in the specified symbol number positions for each subchannel, as shown in FIG. 17. This reference sync/pilot symbol (reference pilot symbol, RPS) is placed on the .sqroot.18 circumference as having a constant amplitude value as shown in FIG. 12. And as shown in FIG. 16, a standardized phase angle is given for each symbol number. From this and the I components and Q components on a receiving end side, the demodulation of the sync/pilot data and information symbols become possible. Normally, when demodulation is performed in the receiving end side, there are errors for the information symbols (IS) and sync/pilot symbols (PS), in that they are off in comparison to the reference-symbol points.
The slot structure is based on 15 millisecond time, as shown in FIG. 17. For the falling main slot structure, the structure is comprised of 60 symbols. The 60 symbols are comprised of three sync symbols, seven pilot symbols, and 50 information symbols. In the rising main slot structure it is comprised of 53 symbols, an AGC preamble and a ramp. The 53 symbols are comprised of three sync symbols, seven pilot symbols, and 43 information symbols. In the rising subslot structure it is comprised of 20 basic slot symbols, an AGC preamble and a ramp. The 20 symbols are comprised of three sync symbols, three pilot symbols, and 14 information symbols. As shown in FIG. 16, the number is counted up with the head slot symbol as symbol number one, and the phase angles for these sync and pilot symbols are set for each sync and pilot number.
The frame structure is based on a 90 millisecond time, with six-slots as the basic unit. This is used for the six-channel multiple time division, multiple-access communication, or TDMA (time division multiple access).
The following is the modulation error .epsilon. calculation method 31: ##EQU1##
The modulation precision for this digital communication method is defined in the formula for the modulation error calculation method 31. In the formula the variables are as follows: i=observed vector number, N=total observed vectors, r0=maximum radius (.sqroot.18) of the signal-space diagram, Vmi=i-th observed vector, Vri=reference vector of the i-th observed vector, V0=origin-offset vector, .alpha.=gain parameter (scalar), and .phi.=phase parameter (scalar). The parameters V0, .alpha. and .phi. should be selected to minimize the modulation precision .epsilon.. The modulation error .epsilon. in FIG. 13 indicates the modulation error for the i-th observed vector.
The actual implementation example is described by referencing FIGS. 10 and 11, for the DMCA modulation precision measurements.
The structure for this device is comprised of a digital conversion component 190, signal generator 183, workstation 184, and analysis software 170, as shown in FIG. 10. This digital-conversion component 190 is comprised of a frequency convertor 192, low pass filter (LPF) 193, AD convertor 194, buffer memory 195, controller 196, and GPIB interface 197. The analysis software 170 is comprised of a channel separator 182, IQ conversion component 173a-173d, route Nyquist filter component 174a-174d, clock-estimation component 175a-175d, carrier-offset estimation components 176a-176d, and modulation-precision error-operation component 177, as shown in FIG. 11.
The measurement signal 191 from the device under test is input to the frequency convertor 192 which is also provided with signals from the high-precision signal generator 183 for the other input. After converting to the desired intermediate frequency, for example 100 KHz, and passing the low frequency signals through the low-pass filter LPF193, the signals are supplied to the AD converter 194.
The AD converter 194 receives the signals, converts the signals into 12-bit digital signals, and stores them to the buffer memory 195. The memory capacity is 1M word, and it can store multiple 90 millisecond frames. The sampling and storage for this AD converter 194 is executed and controlled from the commands received by the controller 196 from the GPIB interface 197. The sampled data is transferred to the memory in the workstation 184 from the buffer memory 195 via GPIB interface 197. Modulation precision is attained by executing the analysis process to the transferred DMCA data.
On the workstation side, the above transferred data is received and analysis processing is executed using the analysis software 170.
The channel separator 172 converts the DMCA data using the high-speed Fourier transform (FFT) to the data on the frequency axis, then separates the M16QAM signals to four-channel subchannel signals (16QAM). Next, each of the IQ conversion component 173a-173d converts with orthogonally into IQ signals for each channel.
Each of the route Nyquist filter component 174a-174d receives these signals and performs route Nyquist filter processing with the roll-odd rate 0.2, then supplies the signals to the clock estimation component 175a-175d.
Each of the clock-estimation components 175a-175d receives these IQ signals and performs a non-linear processing to each subchannel IQ signal to obtain the modulation-signal clock-frequency spectrum. From this spectrum, the estimation of the clock frequency and phase is performed.
Since the symbol points are already determined (as described above), each of the carrier offset estimation components 176a-176d estimates the carrier offset per burst from the predetermined phase and phase differences of the symbol points of the sync and pilot symbols.
The modulation-precision error-calculation component 177 calculates the vector from the symbol points described above. The vector error, clock alignment error, and carrier frequency offset can be calculated with the error vector from the reference vector points shown in FIG. 13. From this error data the vector-error bandwidth, phase waveform, vector-error FFT, eye pattern, constellation, etc., are displayed on the screen.
In actualizing the method with the above-described structure there were inconveniences such as the slow processing speed due to the calculations for FFT calculation processing, etc., being performed by the generic workstation CPU. And because large quantities of AD conversion data are transferred directly to GPIB, which takes time, the processing time can only be measured in minute units each time. Also, the signal generator 183 and workstation 184 would be required externally so the system structure becomes large in scale and expensive.